An anti-fuse ROM integrated-circuit device typically operates from a 5 volt power supply. In order to program the anti-fuse links for a typical anti-fuse ROM, a programming voltage of between 9.5 and 12.5 volts is required. Therefore, the programming circuitry within the ROM integrated-circuit device must be able to provide a minimum programming voltage of 12.5 volts to guarantee that all of the anti-fuse links of a ROM device are properly programmed. A typical external programming voltage applied to the ROM device is 13 volts. When the various memory cells of the ROM device are being programmed, the 13 volt programming voltage is switched within the ROM device to the various memory cells. The switching transistors require voltages higher than 1.25 volts to turn on the switching transistors.
In the prior art, charge pump circuits are very often provided on the ROM device to generate the higher voltages needed for operating the switching transistors. These charge-pump circuits internally produce voltages of 17 volts. These charge-pump circuits therefore require voltage levels which are 4 volts in excess of the 13 volt programming voltage applied to the anti-fuse links.
A problem may arise because of the junction breakdown voltage requirements for the ROM integrated-circuit device. Any internal voltages in the ROM integrated-circuit device must be kept below the junction voltage requirement to guarantee the integrity and performance of the integrated-circuit. Previously, breakdown voltage specifications were not a problem for a semiconductor integrated-circuit fabrication process which has, for example, a feature size of 1.0 micron because the breakdown voltage specification for such a fabrication process was 17 volts.
For newer semiconductor integrated-circuit fabrication processes which have, for example, feature sizes of 0.8 microns, the junction breakdown voltage specification is 16 volts. For a ROM device which uses charge-pump circuits, this means that the voltage at the top of an anti-fuse link cannot exceed 12.0 volts, which is i volts less than the 13 volt programming specification requirement. This problem will be much worse with fabrication processes which use 0.6 micron and smaller feature sizes because these fabrication processes have even lower junction breakdown voltages.
Therefore, the problem with programming of anti-fuse ROM devices with smaller and smaller feature sizes is how to accommodate the junction breakdown voltage requirement for a particular fabrication process while still providing the maximum voltage required to program an anti-fuse link. Thus, a need exists for programming circuitry for anti-fuse ROMS which operates with voltages below the breakdown voltages of their fabrication processes.